In order to melt the scrap material, there have been used electric melting furnaces such as DC or AC arc furnaces or non-electrode type melting furnaces in which the scrap material is charged into a furnace shell preliminarily charged with molten steel and oxygen (O.sub.2) and fuel such as oil or coal are fed to the furnace shell to burn the fuel, thereby heating and melting the scrap material.
Conventionally, a furnace roof is opened when scrap material is to be charged into a melting furnace of the type described above.
FIG. 1 schematically illustrates a conventional melting furnace in the form of a DC arc furnace which comprises a furnace shell 2 with a bottom electrode 1 (anode) at its bottom, a furnace roof 3 for selectively opening and closing an upper opening of the furnace shell 2, an upper electrode 4 (cathode) vertically extending through the roof 3 coaxially of the latter, an exhaust duct 5 connected to the roof 3, a roof opening-and-closing device 6 which supports the roof 3 for vertical and pivotal movements of the latter and an electrode raising-and-lowering device 7 mounted on the roof opening-and-closing device 6 for vertically moving the upper electrode 4. The furnace shell 2 is rockably supported by a lower supporting stand 8 through a curved rocker 9. When the furnace shell 2 is tilted in a direction perpendicular to the Figure, molten steel 10 in the furnace shell 2 is poured through a spout (not shown) into a ladle 12 on a carriage 11.
A material preheating device (not shown) installed separately from the furnace shell 2 receives high-temperature exhaust gases from the shell 2 through the exhaust duct 5 to preheat the scrap material, the exhaust duct 5 being midway provided with combustion cylinder means (not shown) for complete combustion of unburned CO gas. The scrap material 13 which has been preheated by the material preheating device is charged at the top of the furnace shell 2 with the roof 3 being opened, using a bucket or the like.
More particularly, in a melting operation of the scrap material 13, the roof opening-and-closing device 6 lifts up the roof 3 and swings it outwardly to open the top of the furnace shell 2. Using a bucket or the like, the scrap material 13 which has been preheated by the preheating device is charged into the furnace shell 2. Then, the top of the furnace shell 2 is closed with the roof 3 and the gases in the furnace shell 2 is sucked by the exhaust duct 5. The upper electrode 4 is lowered gradually to a predetermined position and then the electrodes 1 and 4 are energized to generate and maintain an arc to melt the charged scrap material 13.
In this case, the substantial weight of the scrap material 13 is slight in comparison with volume thereof and a predetermined or desired quantity of molten metal cannot be obtained by only one charge. Therefore, after the completion of one charge, the electrodes 1 and 4 are de-energized and the furnace roof 3 is opened. New scrap material is charged into the furnace shell 2 and the melting operation is repeated in the manner described above. Then, the furnace shell 2 is tilted to pour the molten metal 10 through the spout into the ladle 12.
In the melting furnace of the type described above, whenever the scrap material is to be charged into the furnace shell 2, the furnace roof 3 must be opened. Such opening of the roof 3 causes various adverse problems. High-temperature exhaust gases are dispersed outside of the furnace and a considerably large quantity of heat is dissipated into the surrounding atmosphere, resulting in great heat loss; a considerably large quantity of dust is spread; the charging of the scrap material causes noise; upon charging of additional scrap material 13 the power source must be de-energized to stop the melting operation; and because of the arc time loss, the temperature of the molten metal 10 drops so that melting efficiency is substantially decreased.
In order to overcome the above-mentioned problems in the melting furnace, a melting furnace as shown in FIG. 2 has been proposed which is exemplarily in the form of a DC arc furnace with a scrap material preheating and charging apparatus 15. In order to permit charging of the scrap material 13, the furnace shell 2 is provided at its side with a material charging opening 14. The opening 14 is communicated with the scrap material preheating and charging apparatus 15 which extends laterally of the furnace shell 2. The device 15 is provided with a material preheating section 17 having a slide type material delivery device 16 at a laterally extending bottom of the device 15 as well as an exhaust duct 19 which is connected to an upper portion of an extension of the preheating section 17 and which is also connected to an exhaust device 18 having a heat exchanger, a dust collector, a suction fan and so on. The preheating section 17 is provided with burner means 20 to further heat the scrap material 13 which has been preheated by the high-temperature exhaust gases 24 and is being transported by the material delivery device 16.
A further example of a conventional scrap material preheating and charging apparatus is a rotary kiln type scrap material preheating and charging apparatus 22 as illustrated in FIG. 3 in which a rotatably driven preheating cylinder 21 is communicated at its lower end with a material charging opening 14 on the furnace roof 3 so as to have a predetermined angle of inclination. In this case, the scrap material 13 is fed to an upper end opening (i.e. the right end in the Figure) of the rotating cylinder 21 and is transported by rotation and inclination of the cylinder 21 toward the opening 14. This example is also provided with burner means 23 disposed above the opening 14 for heating of the scrap material.
FIG. 4 illustrates an example of a conventional, non-electrode type melting furnace. Molten steel is previously charged into a furnace shell 25 and the oxygen 02 and fuel such as oil or coal is fed to the furnace so that the scrap material is heated and melted by combustion of the fuel. The furnace shell 25 is opened at its top to permit the discharge of high-temperature exhaust gases 24 from the shell 25. In this example, a vertical, material preheating and charging apparatus 32 comprises a material preheating section 29 having a plurality of slide gates 26, 27 and 28, each comprising a pair of gate sections slidable toward and away from each other to selectively close and open the passage of the exhaust gases 24 in the device 32. The device 32 further comprises an exhaust duct 30 opened above the material preheating section 29 and a sealing damper 31 located at a top end of the device for selective closing and opening the device so as to charge the scrap material 13 into the device.
In the above-described scrap material preheating and charging apparatus 15 or 22 shown in FIG. 2 or 3 having the material preheating section or cylinder 17 or 21 extending laterally of the furnace shell 2, the high-temperature exhaust gases 24 contact mainly with an upper surface of the scrap material 13 being transported in the form of a bed within the material preheating section or cylinder 17 or 21, resulting in considerably low heat exchange efficiency of the exhaust gases 24 with the scrap material 13. To compensate for this, the scrap material preheating and charging apparatus 15 or 22 is lengthened and the burner means 20 or 23 are required.
In the vertical, material preheating and charging apparatus 32 shown in FIG. 4, the slide gates 26, 27 and 28 and especially the lowermost slide gate 26 is directly exposed to the high-temperature exhaust gases 24 (about 1500.degree. C.) from the furnace shell 25 and the slide gates 26, 27 and 28 must support the scrap material 13 which is heavy, so that the slide gates must be designed and constructed such that they can satisfactorily operate under these severe conditions. Thus, it has been difficult to use such vertical, material preheating and charging apparatus especially in view of economy and from the standpoint of maintenance and repair.
The present invention was made to overcome the above and other problems encountered in the prior art and has for its object to provide a scrap material preheating and charging apparatus which can preheat, at higher exchange efficiency, scrap material by high-temperature exhaust gases from a furnace shell, can prevent moving parts from being directly exposed to the high-temperature exhaust gases from the furnace shell to thereby enhance strength of the apparatus and can substantially continuously charge the preheated scrap material into the furnace shell.